1. Field of the Invention
The present invention relates to a valve mechanism used in the intake and the exhaust systems of an engine.
2. Background of the Invention
Each of an intake and exhaust systems that an engine is equipped with includes a valve mechanism. The valve mechanism includes a valve, a shaft (a valve stem) that is supported by a cylinder head and that moves in the axial direction of the valve shaft to open and close the intake, or the exhaust, port. The valve mechanism also includes a spring retainer attached to the upper end portion of the valve shaft, and a valve spring disposed between the spring retainer and the cylinder head to push up the valve in such a direction as to close the port. The valve mechanism additionally includes a tappet (which is also called a valve-lifter), provided above the spring retainer while being capable of freely moving in the axial direction of the valve shaft. Also included is a cam mechanism, which, against the biasing force of the valve spring, pushes the valve down (or actually, pushes directly the tappet down) in such a direction as to open the port.
There is a space that exists between the top face of the tappet and the cam face in a state where the port is completely closed by the valve. The distance of the space is called valve clearance. The valve clearance in the above-described valve mechanism can be adjusted by an inner shim that is set between the upper end portion of the valve shaft and the under face of the tappet. The inner shim is a disc-shaped metal member. The valve clearance is adjusted by altering the distance between the upper end portion and the under face. To alter the distance, the thickness of the inner shim has to be changed by replacing an inner shim with another shim having a different thickness. A hole is formed in the spring retainer to attach a shim (hereafter, the hole is referred to as shim-attachment hole). While the upper end of the valve shaft serves as the bottom base of the shim-attachment hole, the shim-attachment hole has an open end on the upper side. To attach the inner shim, the inner shim is fitted into the shim-attachment hole from above. At this time, the bottom face of the inner shim is in contact with the top face of the valve shaft, while the top face of the inner shim is in contact with the bottom face of the tappet (for example, see Japanese Patent Application Laid-Open No. 2001-65318).